This project studies the pathophysiological and hormonal regulation of membrane transport systems and the mechanisms by which age-dependent changes perturb physiological control systems and, thus, contributes to the failure to maintain homeostasis in the aged. Notable scientific achievements include the findings that: (1) glucocorticoids, but not mineralocorticoids, induce an increase in amiloride-sensitive sodium-proton exchange activity and a decrease is sodium-dependent phosphate uptake in renal membrane vesicles, findings consistent with the effects of glucocorticoids in stimulating acid secretion and causing phosphaturia; (2) in chronic uremia, the decreased phosphate transport is not attributable solely to an increased PTH level, uremic rats adapt normally to a low phosphorus diet, and the low phosphorus diet can reverse the phosphate transport decrement in uremic animals; (3) monoclonal antibodies reveal the presence of an antigenically distinct enzyme (maltase) in senescent animals; and (4) in established and primary kidney cell cultures, hormones and drugs that activate adenylate cyclase and protein kinase inhibit phosphate uptake.